Lighting device for lighting a liquid crystal display

ABSTRACT

A lighting device has a light source for emitting lighting light for illuminating a display, and a light guide plate for changing the direction of the lighting light to the display. A polarization separation film is provided for separating polarization of the lighting light into P-polarized light and S-polarized light for illuminating the display.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a lighting device for lighting aliquid crystal display (LCD).

[0002] A color LCD is used in a portable electronic device such as aportable telephone in recent years. A lighting device for such a colorLCD comprises, for example a light source (LED), a lighting panel forapplying light from the LED to the color LCD. The light source for thecolor LCD is required to have high luminance and a low consumption powerin order to elongate the life of a battery for the light source.

[0003] In a conventional lighting device, a polarization filter isdisposed between the lighting device and the LCD in order to absorbunnecessary polarization components. As a result, considerable quantityof light is uselessly absorbed.

[0004] Japanese Patent Application Laid Open 11-96819 discloses alighting device which may satisfy such abhorrent requirements. Thelighting device comprises a light source, a lighting panel having anincident surface corresponding to the light source, a lower reflectionsurface, an upper discharge surface, and a reflection plate disposedbelow the lighting panel. In the lighting device, there is furtherprovided a polarization dividing plate, a polarized light beam splitter,and others. Thus, the device is complicated in construction because of alarge number of parts, which causes the manufacturing cost to increase.

SUMMARY OF THE INVENTION

[0005] An object of the present invention is to provide a lightingdevice which may be simple in construction and manufactured at a lowcost.

[0006] According to the present invention, there is provided a lightingdevice comprising a light source for emitting lighting light forilluminating a display, a light guide plate for changing the directionof the lighting light to the display, a polarization separation film forseparating polarization of the lighting light into linearly polarizedlight for illuminating the display.

[0007] The lighting device further comprises at least one polarizationdirection changing film for changing the polarization of transmittinglight into random polarization.

[0008] A microprism having a plurality of scalene prisms for reflectinglight so as to reduce an incident angle on a surface of the device.

[0009] The polarization separation film is provided for separatingpolarization of the lighting light into P-polarized light andS-polarized light.

[0010] These and other objects and features of the present inventionwill become more apparent from the following detailed description withreference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0011]FIG. 1 is a side view of a lighting device generally showing thedevice of the present invention;

[0012]FIG. 2 is a side view of a lighting device according to a firstembodiment of the present invention;

[0013]FIG. 3 is a side view of a second embodiment of the presentinvention;

[0014]FIG. 4 is a side view of a third embodiment of the presentinvention;

[0015]FIG. 5 is a graph showing reflectances of a three-layerpolarization separation film which is made so as to increase thereflectance of S-polarized light;

[0016]FIG. 6 is a graph showing reflectances of a polarizationseparation film made by surface plasmon.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0017]FIG. 1 is a side view of a lighting device generally showing thedevice of the present invention, and FIG. 2 is a side view of a lightingdevice according to a first embodiment of the present invention.

[0018] Referring to FIG. 1, a lighting device 1 comprises an LED 2 as alight source, a lighting panel 3 corresponding to the LED 2, a prismsheet 4 disposed above the lighting panel 3 for arranging the lightdischarged from an upper discharge surface of the lighting panel 3, adiffusion sheet 5, and a reflection plate 6 disposed under the lightingpanel 3. Light is discharged from the diffusion sheet S in the directionshown by the arrow so as to illuminate the LCD (not shown).

[0019] The present invention is to improve the lighting panel 3.Referring to FIG. 2, a lighting device 3A comprises a light guide plate10 corresponding to the LED 2, a polarization separation film 11provided on the light guide plate 10, a cover 12 on the polarizationseparation film 11, a polarization direction changing film 13 on theunderside of the light guide plate 10, and a microprism 14 on theunderside of the polarization direction changing film 13. The reflectionplate 6 is disposed under the microprism 14. The polarization directionchanging film 13 may be disposed on the underside of the microprism 14or on the reflection plate 6.

[0020] The polarization separation film 11 transmits only light P of theP-polarization light, which is parallel to the incident surface, in therandom polarization lights R emitted from the LED 2, and reflects lightS of S-polarization light, which is perpendicular to the incidentsurface. The polarization separation film 11 is a three-layer filmcomprising a central layer of MgO having a refractive index of 1.75 andthickness of 95.6 nm and a pair of outer layers each of which is a filmof CaF₂ having a refractive index of 1.24 and thickness of 186 nm, inthe case of Brewster angle of 40 degrees. In the case of the three-layerfilm, as shown in FIG. 5, since the incident angle is between 36° and40°, it is possible to separate the P-polarization light andS-polarization light.

[0021] The polarization direction changing film 13 is provided forchanging polarization direction of light, and formed by a λ/4 phasefilter or a diffusion filter which has slight refractivity, making adifference of λ/4 in optical thickness between crossing polarizationcomponents.

[0022] The cover 12 on the polarization separation film 11 is formed bythe material for forming the film 11. It is preferable to use the samematerial as the light guide plate 10.

[0023] In the lighting device 3A, only light P of P-polarization in thelight R of random polarization emitted from the LED 2 transmits thepolarization separation film 11, and the light S of S-polarization isreflected. When the incident angle of the light P to the upper surfaceof the cover 12 is smaller than the critical angle, the light dischargesfrom the cover. When the incident angle is larger than the criticalangle, the light P is reflected. The reflected lights P and S transmitsthe polarization direction changing film 13 of the λ/4 phase filter sothat the phase difference between crossing polarization componentsbecomes π/2, thereby becoming random polarization. The incident angle ofthe random polarization to the microprism becomes larger than thecritical angle, the light transmits the microprism 14 as shown in FIG. 2and is reflected by the reflection plate 6. By transmitting thepolarization changing film 13, the light S of S-polarization becomeslight P of P-polarization component. The P-polarization light transmitsthe lighting device 3A to illuminate the LCD.

[0024] The polarization separation film 11 may be formed by three-layerfilm of surface plasmon. The polarization separation film 11 generatesevanescent wave which excites the surface plasmon, thereby transmittingthe light of P-polarization component and reflecting the light ofS-polarization component.

[0025]FIG. 3 is a side view of a second embodiment of the presentinvention. A lighting device 3B comprises a light guide plate 15, amicroprism 16 formed on the lower surface of the light guide plate 15, apolarization separation film 17 formed on the underside of themicroprism 16, an intermediate member 19 on the underside of thepolarization separation film 17, and a polarization direction changingfilm 18 formed on the underside of the intermediate member 19.

[0026] The light P of P-polarization component in the light R emittedfrom the light source 2 transmits the polarization separation film 17,and the light S of S-polarization component in reflected by the film 17,The light S reflected by the film 17 is transmitted from the light guide15. On the other hand, the light P is reflected by the reflection plate6 to be returned to the light guide plate 15. Since the light P passesthe polarization direction changing film 18 two times, the polarizationplane is rotated 90°, so that the light P is changed to light S ofS-polarization component. The light S of incident angle smaller than thecritical angle transmits the light guide plate 15 for the illuminationof the LCD. The light S of large incident angle is repeatedly reflectedby the upper surface of the light guide plate 15 and the longer side ofthe prism of the microprism 16, which corresponds to the light source 2,so that the incident angle gradually reduces to a incident angle smallerthan the critical angle, thereby discharging from the light guide plate15.

[0027]FIG. 4 is a side view of a third embodiment of the presentinvention. A lighting device 3c comprises a light guide plate 20, afirst polarization direction changing film 21 on the upper surface ofthe plate 20, a first cover 22 on the polarization direction changingfilm 21, a second polarization direction changing film 25, a secondcover 26 on the film 25, a polarization separation film 23 on theunderside of the plate 20, and microprism 24 on the film 23.

[0028] In the lighting device 3C, the light t from the LED 2 isseparated to the light P and the light S, the light P transmits thepolarization separation film 23, and the light S is reflected by thefilm. The reflected light S transmits the polarization directionchanging film 21, thereby being changed to a random polarization. Therandom polarization is reflected by the upper surface of the first cover22 because of large incident angle, and transmits the polarizationdirection changing film 21 to be changed to light P. The light P passesthe polarization separation film 23 and is reflected by the microprism24 and passes the polarization separation film 23 again. The light Ppasses the polarization direction changing film 21, thereby beingchanged to random polarization. The random polarization transmits thefirst cover 26.

[0029] On the other hand, the light P which first passes thepolarization separating film 23 is reflected by the microprism 24 andpasses the polarization separation film 23. The light P is changed tothe random polarization by passing the polarization direction changingfilm 21. The random polarization is changed to light P of P-polarizationcomponent when passing the second polarization direction changing film25, and discharged from the second cover 26.

[0030] In accordance with the present invention, a large quantity oflight emitted from the light source is changed to linearly polarizedlight necessary to illuminate the LCD. Therefore, the light from thelight source is effectively used without wasting electric power of thebattery.

[0031] While the invention has been described in conjunction withpreferred specific embodiment thereof, it will be understood that thisdescription is intended to illustrate and not limit the scope of theinvention, which is defined by the following claims.

What is claimed is:
 1. A lighting device comprising: a light source foremitting lighting light for illuminating a display; a light guide platefor changing the direction of the lighting light to the display; apolarization separation film for separating polarization of the lightinglight into linearly polarized light for illuminating the display.
 2. Thelighting device according to claim 1 further comprising at least onepolarization direction changing film for changing the polarization oftransmitting light into random polarization.
 3. The lighting deviceaccording to claim 1 further comprising a microprism having a pluralityof scalene prisms for reflecting light so as to reduce an incident angleon a surface of the device.
 4. The lighting device according to claim 1wherein the polarization separation film is provided for separatingpolarization of the lighting light into P-polarized light andS-polarized light.